The present invention relates to a gas turbine combustor for combusting premixed fuel in a fuel lean state which is obtained by adding air to fuel and an operating method thereof, and more specifically, to a gas turbine combustor capable of effectively lowering concentration of NOx contained in the exhaust gas from a gas turbine and an operating method thereof.
In general, a gas turbine power generation plant has a plurality of gas turbine combustors interposed between an air compressor and a gas turbine and creates a combustion gas by the gas turbine combustors by adding a fuel to a compressed air guided from the air compressor. The combustion gas is guided into the gas turbine and an expansion work is executed and a generator is driven by making use of the rotational torque obtained by the expansion work.
Incidentally, recent gas turbine power generation plants are required to increase a generated power in addition to the increase of a fuel efficiency and, for this purpose, the combustion gas temperature at a gas turbine inlet is increased so as to increase the power of the gas turbine by increasing the temperature of the combustion gas created by the gas turbine combustor.
However, various restrictions are imposed on the gas turbine combustor by the increase of the combustion gas temperature at the gas turbine inlet and one of them is an environment problem relating to a NOx concentration.
The NOx concentration directly depends on the temperature increase of the combustion gas, and as the temperature of the combustion gas is more increased, the concentration thereof is more increased. That is, when the combustion gas is created by the mixture of fuel and air, as an equivalent ratio (ratio of a fuel flow rate to an air flow rate) approaches a value of 1, the temperature of the combustion gas is more increased and the nitrogen contained in the air is bonded to a larger amount of oxygen by the action of the reaction heat resulting from the temperature increase to thereby increase the NOx concentration.
There is available a lean premixing combustion system in the gas turbine combustor as a method of lowering the generation of NOx which burns fuel in a fuel lean state by previously mixing air with the fuel. According to such combustion system, since the fuel itself has been already made to the lean state, when a combustion gas is created, the peak temperature of the combustion gas can be suppressed as compared with a conventional diffusing combustion system and a NOx reduction ratio of about 20% can be ordinarily achieved.
However, as shown in FIG. 19, it is difficult for the lean premixing combustion system to control the equivalent ratio when the combustion gas is created. When the equivalent ratio is low, a combustion efficiency is lowered and the generation of uncombusted components such as CO, UHC (uncombusted hydrocarbon) etc. is increased, and sometimes, a flame blow out phenomenon rises, whereas when the equivalent ratio is high, the amount of NOx generated is abruptly increased. As a result, the range of combustion operation in which a low NOx state can be stably maintained for a long time is very narrow.
Recently, there have been proposed many combustion systems which use diffusing combustion and premixing combustion simultaneously as a technology which further develops the lean premixing combustion system, the systems being arranged such that a diffusing combustion zone is formed to the head portion of a combustion chamber, a premixing combustion zone is formed downstream side the diffusing combustion zone, a diffused combustion gas is created by charging the fuel into the diffusing combustion zone and a premixed combustion gas is created by charging the premixed fuel into the premixing combustion zone. One of the diffusing/premixing combustion systems is disclosed in Japanese Patent Laid-open Publication No. HEI 7-19482.
The prior art technology further reduces NOx by partially premixing pilot fuel for maintaining flame to thereby reduce diffused combustion by which a lot of NOx is generated, in addition to a matter that the main fuel for creating the combustion gas for driving the gas turbine is premixed.
As shown in FIG. 18, a gas turbine combustor according to the prior art technology is arranged such that a diffusing combustion zone 2 is formed to the head portion in a combustor inner cylinder 1, a premixing combustion zone 3 is formed downstream of the diffusing combustion zone 2, and a pilot fuel injection unit 6 for charging a pilot fuel A is disposed to the diffusing combustion zone 2 and a main fuel injection unit 16 for charging a main fuel C is disposed to the premixing combustion zone 3, respectively.
The pilot fuel injection unit 6 includes a diffusing combustion nozzle unit 4 at the center of the combustor inner cylinder 1 and a premixing combustion nozzle unit 5 to the outside of it.
The diffusing combustion nozzle unit 4 is partitioned into a first diffusing combustion nozzle unit 7 for charging a fuel al into the diffusing combustion zone 2 to maintain flame until a low load is imposed on the gas turbine and a second diffusing combustion nozzle unit 8 for charging a fuel a2 into the diffusing combustion zone 2 to maintain the flame in place of the first diffusing combustion nozzle unit 7 when an intermediate load is imposed on the gas turbine. Further, an air passage 9 is formed to the diffusing combustion nozzle unit 4 so as to concentrically surround the first and second diffusing combustion nozzle units 7 and 8, and a swirler 10 is disposed to the outlet end of the air passage 9 to thereby apply a swirling flow to the fuels al and a2 which are injected from the first and second diffusing combustion nozzle units 7, so that a circulating flow is formed in the diffusing combustion zone 2 to more securely maintain the flame.
The premixing/diffusing combustion nozzle unit 5 disposed outwardly of the diffusing combustion nozzle unit 4 is arranged such that when a fuel b which is used as a combustion gas for driving the gas turbine as well as a combustion gas for maintaining the flame is charged into the diffusing combustion zone 2 through a header 11, the nozzle unit 5 mixes the fuel b with the swirling air supplied from a swirler 12 in a premixing zone 13 and injects it into the diffusing combustion zone 2 as the premixed fuel in a lean fuel state and when the premixed fuel is injected, it is made to a circulating flow which is larger than the circulating flow in the first and second diffusing combustion nozzle units 7 and 8.
On the other hand, the main fuel injection unit 16 for charging a fuel c into the premixing combustion zone 3 is composed of a main fuel nozzle unit 14 and a premixing duct 15 and when the fuel c is injected from the main fuel nozzle unit 14 through a header 18, the main fuel injection unit 16 mixes the fuel c with the compressed air 17 from an air compressor, not shown, in the premixing duct 15 and injects the fuel c as a premixed fuel in a lean fuel state into the premixing combustion zone 3 to thereby create a combustion gas for driving the gas turbine using the combustion gas of the pilot fuel injection unit 6 as a pilot flame.
As shown in FIG. 19, a method of charging and distributing the fuel injected from the pilot fuel injection unit 6 into the diffusing combustion zone 2 and the fuel injected from the main fuel injection unit 16 into the premixing combustion zone 3 is performed in a manner such that while the load on the gas turbine, which is in start-up operation, is zero, the fuel al of the first diffusing combustion nozzle unit 7 is charged into the diffusing combustion zone 2. When the gas turbine is rotated 100% in a no load state, the fuel a2 of the second diffusing combustion nozzle unit 8 and the fuel b of the premixing/diffusing combustion nozzle unit 5 are simultaneously charged into the diffusing combustion zone 2. When the gas turbine is in an intermediate load state, the charge of the fuel al of the first diffusing combustion nozzle unit 7 is stopped and the fuel c of the main fuel injection unit 16 is charged into the premixing combustion zone 3 in place of it. When the load on the gas turbine is made to 100%, the ratio of the fuel c to the entire fuel flow rate is set to 70%-80%. Further, it is to be noted that the fuel a2 of the second diffusing combustion nozzle unit 8 at the time is as small as 2-5% which is set to the entire fuel flow rate and it is secured to maintain the flame.
As described above, the conventional gas turbine combustors suppress the generation of the NOx by partially premixing the fuel injected from the pilot fuel injection unit 6 into the diffusing combustion zone 2 as the flame maintaining combustion gas by paying attention to the diffusing combustion by which a large amount of the NOx is generated.
However, since the recent gas turbine power generation plants search for the power and thermal efficiency of the gas turbine which are higher than those achieved at present, a countermeasure for reducing the NOx is more required to cope with the increase of a combustion gas temperature. To maintain the NOx concentration which is lower than that regulated by the present law over the entire operating range from the low load operation to the 100% load operation of the gas turbine, it is required to develop a gas turbine combustor which further reduces the concentration of the NOx generated in the diffusing combustion.
Although the conventional gas turbine combustor shown in FIG. 18 partly executes the premixing of the pilot fuel injection unit 6, it is encountered with difficulty in the development of the premixing of the first diffusing combustion nozzle unit 7 and the second diffusing combustion nozzle unit 8. This is because that since the first diffusing combustion nozzle unit 7 and the second diffusing combustion nozzle unit 8 are provided to stably secure the combustion gas for the flame, when the premixing is executed to these units, there is caused a great factor by which the flame is blown out. When a diffused fuel is supplied into a single large combustion chamber in a small flow rate, a diffusing combustion zone is disturbed by the great disturbance of the premixing combustion zone 3 for the pilot premixed flame and the main premixed flame, by which the flames are made unstable and blown out.
It will be necessary to carry out a control such that when a load is shut off, the premixed fuel is shut off and the diffused fuel restricted to a small amount is increased accordingly. However, since the flow rate of the diffused fuel is not immediately increased due to the volume of a piping from a control valve to a diffusing nozzle injection valve, a premixed flame is misfired by the reduction of the premixed fuel before the flow rate of it increased, an amount of air being supplied increases instantaneously and the air/fuel ratio in the diffusing combustion unit is reduced. At the same time, the disturbance of a cold gas is caused also in the diffusing combustion unit by the misfire of the premixed flame and the diffused flame is blown out. As a result, when the diffused fuel is reduced to lower the NOx, blowing out is liable to be caused in ordinary operation as well as when the load is shut off.
Although a plurality of the gas turbine combustors, for example, eight sets are interposed between the air compressor and the gas turbine, an igniter is provided with one or two of them and the flame generated by the ignition of the igniter is sequentially propagated to the other gas turbine combustors. In this case, even if a combustion chamber is partitioned to a small size at the center of the gas turbine and fuel is supplied thereinto and ignited, only the center of the gas turbine is made to a high temperature by a resulting flame and the flame is not sufficiently propagated to a flame propagation pipe and thus the propagation thereof to the other gas turbine combustors is delayed.